Power control is an important consideration in the design of electronic devices of all types. This is particularly true for portable electronic devices, such as cellular telephones, personal digital assistants and digital cameras. Designers of these devices are faced with the need to continuously increase battery life while continuously increasing the range and type of functions provided. This is dramatically illustrated by the continuing evolution of cellular telephones. Originally equipped with large batteries that provided minutes of operation, cellphones have evolved to the point where a small battery (smaller than the phone in almost all cases) provides days of operation. At the same time, cellphones have continued to become more feature packed to the point that many now include formerly unrelated devices such as digital cameras or PDAs.
To control power, portable electronic devices typically include specialized integrated circuits generally referred to as power control ICs. Different power control ICs are available to function as voltage regulators, current sources and switches. In portable electronic devices, power control IC's are used to operate a wide range of subsystems such as LCDs, LEDs, speakers, and motors. In many cases, power control ICs are implemented as discrete, standalone devices with the result that a single electronic device (e.g., cellphone) may include a large number of different power control ICs. In such cases, controlling the different ICs becomes a significant problem.
Part of this problem is attributable to the fact that small, discrete integrated circuits typically have a very small number of input/output leads. Almost by definition, use of any of these leads for control reduces the number of leads available for other purposes. This is especially problematic for ICs that have complex control interfaces, such as multi-bit control registers. Of course, even where adequate leads are available, routing large numbers of control signals adds cost and complexity to portable electronic devices. In the competitive world of consumer electronics, this can be a serious disadvantage.
In some cases, it is possible to use analog signals to reduce the number of leads required for control purposes. This is particularly true where a function or output is controlled over a range of values (such as a voltage output). In cases of this type, a single analog input can be used to control the function or output. Unfortunately, the use of analog signally is inherently error prone and not always easy to calibrate. It is also true that many microprocessors lack analog outputs, making implementation of analog control signals even more difficult.
Digital control systems are also possible, and may be implemented to pass data serially or in parallel. The digital nature of these systems increases their compatibility with microprocessors. However, even serial implementations typically require at least two (and often three) inputs leads. For where input leads are scarce, the use of two to three leads may be difficult to accommodate. In other cases, the use of two or three input signals per power control IC may create complex routing problems between the ICs and their controlling entity.
For these reasons and others, there is a need for an interface that may be used to control stand-alone power and other IC types. Ideally, this interface would be able to accommodate a wide variety of control needs and be scaleable to many levels of complexity. Minimal pin use is also desirable, with the ideal being use of a single pin that may optionally be shared with another function.